CI: Test installing from git archive

[effigies]

Related to #786. It looks like Versioneer doesn't handle archives that don't include tags very well, so I'm verifying the usefulness of the current setup, and we'll need to find a way to make this work with Versioneer, if it's worth moving to Versioneer at all at this point.

[codecov]

Codecov Report

Merging #791 into master will decrease coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #791      +/-   ##
==========================================
- Coverage   90.02%   89.87%   -0.15%     
==========================================
  Files          94       94              
  Lines       12018    12018              
  Branches     2136     2136              
==========================================
- Hits        10819    10801      -18     
- Misses        852      868      +16     
- Partials      347      349       +2

Impacted Files	Coverage Δ
nibabel/environment.py	75% <0%> (-20%)	⬇️
nibabel/pkg_info.py	65.51% <0%> (-13.8%)	⬇️
nibabel/openers.py	78.22% <0%> (-3.23%)	⬇️
nibabel/casting.py	85.53% <0%> (-0.86%)	⬇️
nibabel/dft.py	80.48% <0%> (-0.61%)	⬇️
nibabel/nifti1.py	91.17% <0%> (-0.31%)	⬇️

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[effigies]

Does anybody actually do this? We have the machinery though, so I figure we should test it until we remove it.